Safety shutoff

ABSTRACT

CIRCUITRY INITIATES TWO SHORTER AND LONG TIMED BUT VARIABLE INTERVALS WHEN A FURNACE THERMOSTATE SWITCHES TO CALL FOR HEAT. THE CIRCUITRY EFFECTS THE SWITCHING TO INITIATE COMBUSTION AT THE END OF THE SHORTER TIMED INTERVAL; BUT, IN THE EVENT THAT NO COMBUSTION HAS COMMENCED BY THE END OF THE LONGER TIMED INTERVAL, THE CIRCUITRY PREVENTS THE COMBUSTION THEREAFTER UNTIL A RESET OPERATION HAS BEEN PREFORMED. PREFERABLY THE RESET IS MECHANICAL. IF COMBUSTION START BEFORE THE ENT OF THE LONGER TIMED INTERVAL, THE TIMIING OF THE LONGER TIMED INTERVAL CEASES.

H. D. JULINOT SAFETY SHUTOFF May 30, 1972 2 Sheets-Sheet 1 Filed July16, 1970 Inventor HEL MU] 0. JUL //V07 Attorney May 30, 1972 H. D.JULINOT SAFETY SHUTOFF Filed July 16, 1970 2 Sheets-Sheet 2 7b Re/qy P ETherm/3m" 5% Inventor HEL/VUTD JUL/N07- A Home y United States Patent3,666,392 SAFETY SHUTOFF Helmut D. Jnlinot, Toronto, Ontario, Canada,assignor to International Telephone and Telegraph Corporation,

New York, N.Y.

Filed July 16, 1970, Ser. No. 55,393 Claims priority, applicationCanada, Mar. 2, 1970, 076,241 Int. Cl. F23n 5/14 U.S. Cl. 431-78 4Claims ABSTRACT OF THE DISCLOSURE Circuitry initiates two shorter andlonger timed but variable intervals when a furnace thermostat switchesto call for heat. The circuitry effects the switching to initiatecombustion at the end of the shorter timed interval; but, in the eventthat no combustion has commenced by the end of the longer timedinterval, the circuitry prevents the combustion thereafter until a resetoperation has been performed. Preferably the reset is mechanical. Ifcombustion starts before the end of the longer timed interval, thetiming of the longer timed interval ceases.

BACKGROUND OF THE INVENTION This invention relates to controls forequipment and, more particularly to an automatic control for shuttingdown equipment such as an oil or gas furnace when the burner fails toignite.

It will be appreciated that when a thermostat calls for ignition andcombustion and if such ignition does not take place at the time calledfor or within a reasonable interval thereafter, many disadvantageoussituations can rise, including the danger of oil spreading and theconsequent danger of fire and/or explosion.

SUMMARY OF THE INVENTION Therefore, it is an object of the invention toprovide a furnace control circuit, where if ignition and combustion arecalled for by the thermostat but do not occur within a predeterminedinterval, the necessary switching will be performed to prevent ignitionthereafter. Means are preferably provided to ensure as far as possiblethat the prevention of combustion continues until the causes of theoriginal combustion delay have been corrected.

It is also an object of the invention to provide furnace controlcircuitry as described in the previous paragraph where, when ignition isprevented electrically and due to a detected delay in such ignition, amechanical reset is required before the control circuitry may againcause ignition and combustion. Thus, electronic release of theprotective circuit is prevented and manipulations of the circuitry forelectric resetting of the control circuitry before the trouble is fixedare inhibited.

When the thermostat calls for heat, it is desirable to know that allelements in the control system are functioning properly. It is an objectof this invention to provide a circuit wherein the more important ofsuch elements must function properly between the period the thermostatcalls for heat and ignition; otherwise, ignition will not take place.

It should be emphasized that the circuit has other failsafe features sothat other elements not affected as described in the preceding paragraphare connected so that their failure will be evident or will also preventoperation of the circuit.

The above-described and other advantages of the invention will be betterunderstood from the following description when considered in connectionwith the accompanying drawings.

3,666,392 Patented May 30, 1972 BRIEF DESCRIPTION OF run DRAWINGSDESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings in FIG. 1, lines10 and 12 are shown carrying standard AC power (usually volts). Lines 10and 12 are shown as connected across ignition device 11 and oil supplymotor 13 in series through normally open contact P1. The indication ofignition device 11 and motor lines for switching on and off by contactP1 are exemplary only. In accordance with conventional arrangements foroil furnace operation, when the circuit is closed, the ignition willintermittently operate until combustion has begun; and the motor willsupply oil from the time of ignition onward.

Lines 10 and 12 are also connected across the primary of the stepdowntransformer T1 whose two ends are connected to a positive line :14through rectifiers 16 and 18 respectively and to a negative line 20through rectifiers 22 and 24 respectively. The center tap of thesecondary of transformer T1 is connected to line 26. Line 14 isconnected through resistor R1 to a line 30. Line 30 is con nected toline 20 through the following elements in series: a thermostat 32, anormally closed contact L1 of a relay L, a relay coil P, a relay coil L,a thermistor 34 to an anode of a silicon controlled rectifier (SCR) 36,a cathode of SCR 35 being connected to line 20.

It will be noted that the relay L and the contact L1 are unconventionaland of a type whereupon if there is sutficient current flow throughrelay L, contact L1 will open in the conventional manner for a relay.But on opening, contact L1 mechanically latches so that it can only beclosed by a mechanical reset. Although the above is shown in FIG. 1, itis shown in detail in FIGS. 2, 3 and 4 and is later described.

It will be noted that the thermistor is a device having a negativetemperature coefi'icient. Thus, on initiation of flow through .SCR 36,primarily self-heating thermistor 34 will gradually heat up; and itsresistance will consequently lower in an amount varying with the timethe intermitttent flow continues with the current in the circuit R1, 32,L1, P, L, 34, and 36 and correspondingly increasing. Resistor 38 isprovided optionally in parallel with thermistor 34 and provides anadjustment of the current through thermistor 34.

If required for desired heating rates, a resistor R2 is provided whichis connected between line 30 and line 26 but is physically juxtaposed tothermostat 32 to supply heating thereto. As stated, thermistor 34' isprimarily self-heating, and the resistor R2 is only used and connectedif auxiliary heating is required due to cold, ambient temperatureconditions or other causes. The levels of operation of relays P and Lare selected .so that they will not operate at the level of initial flowthrough thermistor 34 at environmental temperatures therein. However,relay P is selected and designed to operate at a lower current level(implying a smaller degree and shorter interval of heating of thermistor34) than relay L. The connection between relays P and L is connected toline 26 through the normally open contact P2 of relay P which acts incircumstances to be described, as latching contacts for the relay P. AZener diode 40 and a condenser 42 are connected in parallel with eachother and in parallel with relays P and L and thermistor 34 in series.Zener diode 40 is oriented to provide its design voltage drop forpotentials higher at contact L1 than at the anode of SOR 36. Line 30 isconnected through resistor 44 to the base of a transistor 48, and thebase of transistor 48 is connected to line 20 through a resistor 46.Line 14 is connected through a variable resistor 50 to the emitter oftransistor 48. The emitter of transistor 48 is connected to line 20through a flame detector 52 which would be located in the furnace in thevicinity of a burner flame. The flame detector 52 is a conventional typewhich has a high resistance when no flame exists at the burner and has alow resistance when a flame does exist in the furnace. Although exactvalues will vary with other design parameters, the type of flamedetector which has been used (Type CT 97 manufactured by General Motors)has greater than one megohm resistance when the furnace is off and lessthan one kilohm when the furnace is on. Resistor 50 is chosen along withthe other circuit parameters affecting transistor 48 so that if line 14is at a voltage reflecting an unduly low voltage across lines '10 and12, the transistor 48 will not turn on. This prevents dangers inherentin ignition of the furnace under unduly low voltages across lines and12. The flame detector may, of course, be replaced within the scope ofthe invention by a different type of combustion detector as long asequivalent resistive effects are available for circuit biasing.

In operation then, it will be assumed that the furnace is off withcontact P1 open. Also, it is assumed that the temperature is at thedesired temperature so that thermostat 32 provides an open circuitthrough relays P and L and thermistor 34. Because the furnace is off,the resistance of flame detector 5-2 is high. It will be noted that thepower supply provides full wave supply to lines 14 and 20 and, hence, tothe circuitry of transistor 48. Resistor R1 is selected so that the baseof transistor 48, when turned on, will be low enough to conduct for allvalues of current flow through the circuits, including thermostat 32.Thus, with the furnace ofi, transistor 48 will conduct on eachhalf-cycle. Also, a pulse from the collector of transistor 48 which isdeveloped across resistor 54 will be applied to the gate of SCR 36.However, SCR 36 will not conduct as thermostat 32 provides an opencircuit to the anode thereof.

This situation will continue until the temperature measured bythermostat 32 is below the predetermined setting; at which time, thethermostat will close the circuit between resistor R2 and closed contactL1- hence, through to the anode of SCR 36. Then, SCR 36 is triggered oneach half-cycle by the pulse from transistor 48. Thus, intermittentconduction through a portion of each half-cycle takes place in SCR 36and causes thermistor 34 to gain in temperature. It will be noted thatto the extent that the thermistors own initial temperature is notsufficient in view of ambient conditions at the thermistor 34 and/or tothe extent that the rate of heating is not sufficiently fast, resistorR2 may be provided between line 30 and line 26 to provide auxiliaryheating. In any event, the operating levels of the relays P and L areselected so that initial conduction through the unheated thermistor willnot energize either relay. At a later time determined by the heatingrate of thermistor 34 and its initial and continuing ambienttemperature, the thermistor resistance falls to a sufficiently low valuethat the current level in the series circuit between line 30 and to andincluding SCR 36 is suflicient to operate relay P, which closes normallyopen contacts P1 and P2. Coutact P2 acts to latch in relay P to maintainit energized independently of relay L, thermistor 34, and SCR 36 for aslong as contact L1 and thermostat 32 remain closed. Contact P1 closes toinitiate the operation of the ignition and the motor circuits. Like themotor and ignition circuits shown, the contact P1 may be considered asexemplary.

At the time that the current through relay P is suificient 4 to operateits contacts, it will be noted that relay L has not yet operated.

A timed interval is the time from the initiation of conduction ofthermistor 34 until the current therethrough reaches a level to operaterelay P. The exact length of such interval will vary with the initialtemperature of the thermistor 34.

In accordance with the present invention, the closure of contact P1 is asequential step in the successful operation of the furnace control. Itwill be noted that the device so far is substantially fail-safe in thatin the event of low voltage, transistor 48 would not conduct so that nopulse would be provided to ignite SCR 36. Also, energizing current forrelay P would not be initiated.

Further, it will be noted that if any of relays P or L or thermistor 34have failed open, the circuit will not operate; and the furnace cannotbe ignited. Moreover, should condenser 42 or Zener diode 40 fail, therelays will not operate; and the furnace similarly will not be lighted.If condenser 42 fails open, the ripple through the relays P and L andthermistor 34 will cause noise and will tend to shorten the time atwhich relay L is operated (as hereinafter described) to shut off theignition and motor circuits. Zener diode 40 tends to prevent highvoltage fluctuations through the relays and thermistor. Should it failon short circuit, relay P, inter alia, will not operate, and the furnacewill not light. Should Zener diode 40 fail at open circuit, then thepresence of high current fluctuations will shorten the time of operationof relay L which, as hereinafter described, will operate to prevent orto turn off the ignition and motor circuits as hereinbefore described.Thus, the circuitry is fail-safe.

Where the closure of contact P1 has caused ignition of the furnace, heatwill continue until the resistance of detector 52 falls below the valueat which conduction can take place in transistor 48. This will open orstop conduction in SCR 36, and the heating of thermistor 34 will cease.Thus, relay L cannot operate. However, the operation of the furnace isnot impaired since relay P is latched closed through contact P2. Whenthe heat in the ambient area reaches the preset temperature level,thermostat 32 opens which then opens the latching circuit for relay P,allowing it to open opening contact P2 and disconnecting the contact P1which will turn off the furnace. This constitutes the completion of anormal cycle of operation of the furnace and control.

Going back to the state of the operation where contact P1 has justclosed, let it be assumed that the ignition or the motor or both do notoperate so that with no combustion taking place, a dangerous conditionmay arise. One of these dangerous situations may occur where oil issupplied to the furnace, but no ignition takes place which causes aspread of oil. This is a serious situation if ignition takes place atthat time. In any event, with contact P1 closed, the thermostat 32previously closed, and no ignition, current flow continues throughthermistor 34 on intermittently triggered half-cycles from thetransistor 48. With the heating of thermistor 34, the resistance thereofdecreases, and the current through the thermistor and relay L increases.When the resistance of thermistor 34 decreases to allow sufficientcurrent flow, relay L is operated which opens contact L1thus,disconnecting the latching circuit of relay P and preventing laterignition or combustion in the furnace by opening the contact P1.

When contact L1 has opened, as previously described because of a faultcondition, it is desirable that contacts L1 not be easily reclosed.Thus, contact L1 is provided preferably with means to be mechanicallylooked open. Once opened, they can only be reset mechanically whichprevents the chance or electric reclosing of contact P1.

It will be noted that the circuit is also designed to disconnect themotor and/or ignition circuits in the event that the flame goes outafter proper ignition. In this event with contacts P1, P2, and L1closed, the lack of flame will cause the resistance of detector 52 torise until transistor 48 turns on, supplying pulses to SCR 36.Therefore, with such pulses, conduction resumes through thermistor 34.When thermistor 34 heats to a suflicient degree, relay L operatesopeningcontact L1 (which locks itself open as hereinafter described),de-energizing relay LP, and opening contacts P1 and P2.

It should be noted that resistor 38 will be selected or set at valuestoo high to bring in relays L or P if the thermistor 34 should fail asan open circuit.

In FIGS. 2, 3, and 4, the preferred design for relay L to achieve thedesired feature of manual reset is shown. In these figures, relay L andits connections are conventional. The frame 100 which holds the relay Lalso provides a fulcrum on which armature 104 may pivot. Armature 104 isbiased away from relay L by tension spring 106 attached at one end toarm 108 which projects from armature 104 and at the other end to aprojection from frame 100.

Incorporating arm 108 extends upwardly on each side of fulcrum 102 toprovide base plate 110 located over the relay. Mounting spacers 112 andtop plate 114 are attached to base plate 110 in any desired manner, suchas by rivets 116. Spacers 112 maintain the desired spacial relationshipbetween contact arms 118 and 122 which are conventional. Arms 118 and122 have contact points 124 and 126 respectively.

Contact arm 122 is mechanically linked to armature 104, as shown, formovement therewith. Thus, in the absence of current in relay L, contact126 is held touching contact 124 under the control of armature 104 asbiased by spring 106. Plate 114, approximately over contacts 124 and 126is provided with a downwardly projecting prong 128. Armature 104 isprovided with a spring metal extension 130 having an upwardly turnedfree end 131. End 131 is designed to rest inwardly of prong 128 whencontacts 124 and 126 are closed and is shaped and resilientlyconstructed to move outwardly of prong 128 when extension 130 andarmature 104 move downwardly together on the operation of relay L.

End 131 is further designed in combination with prong 128 that on anyattempt by armature 104 and arm 122 to move upwardly on de-energizationof relay L, it catches on the outside of prong 128 and interlockstherewith to prevent upward movement of contact 126 and movement of arm122 toward closure of contact 126 with contact 124. Thus, contacts 124and 126 cannot be closed electrically once opened. Mechanical means forresetting the relay to the normally closed position of the contacts willnow be described.

A push button 136 is spring-mounted on plate 114 to overlay extension130 in the vicinity of prong 128. A projection 140 from push button 136extends toward extension 130 but is biased (in the absence of pressureon the push button) just out of contact with extension 130. However,with extension 130 bearing against prong 128 as shown in FIG. 3,contacts 124 and 126 may be caused to close by depression of button 136,whereupon extension 130 near prong 128 is depressed. On release ofbutton 136, end 131 moves inwardly of prong 128 whereupon contacts 124and 126 may close under the impulsion of spring 106.

Button 126 may be located in the immediate vicinity of the furnace,requiring attendence thereat before resetting can be performed.

Instead of the thermistor, any device may be used which has self-heatingmeans, a negative temperature coefficient, and operable within thecurrent and time limits required for the two intervals. Where athermistor is used, it is preferred that Model B8-320-01A, 500 ohm NTCmanufactured by Philips Electron Devices be used.

What is claimed is:

1. In a combustible fuel ignition system, the combination comprising:first and second power input leads; a thermostat switch actuable inresponse to the temperature of a space to be heated; a first relayhaving a winding and a pair of mutually engagable normally opencontacts; a second relay having a winding and a pair of mutuallyengagable normally closed contacts; a negative temperature coefficientthermistor; a silicon controlled rectifier having an anode-cathodecircuit including an anode and a cathode, said rectifier also having agate; means responsive to the closure of said first relay contacts tosupply fuel to a combustion space and to ignite the fuel; saidthermostat switch, said second relay contacts, said first and secondrelay windings, said thermistor, and the anode-cathode circuit of saidrectifier being connected in series between said power input leads; aflame detector; a safety control circuit connected from said power inputleads and from said detector to said rectifier gate to control thecurrent flowing in the anode-cathode circuit of said rectifier inaccordance with the output of said detector, said second relay having aconstruction requiring manual closing of said contacts thereof afterthey have been opened, closure of said thermostat switch causingimmediate energization of said first relay and conduction in theanode-cathode circuit of said rectifier, said thermistor beingconstructed so as to heat during said rectifier conduction, said secondrelay and said thermistor having a construction so as not to energizesaid second relay when said first relay is first energized, said controlcircuit limiting the current through said rectifier and preventingenergization of said second relay if fuel ignition is detected by saiddetector within a predetermined time interval, said control circuitallowing sufiicient current to flow through said rectifier and saidthermistor to energize said second relay and open the said contactsthereof when fuel ignition is not detected by said detector within saidpredetermined time interval, energization of said second relay causingdeenergization 'of said first relay, opening of said first relaycontacts and the shutoff of said fuel supply to prevent a hazardousbuildup of fuel in the absence of the ignition thereof.

2. The invention as defined in claim 1, wherein a capacitor is connectedin parallel with at least said second relay.

3. The invention as defined in claim 1, wherein a Zener diode isconnected in parallel with the series connection of said first andsecond relay windings and said thermistor, said diode being poled in adirection opposite that from the cathode to the anode of said rectifier.

4. The invention as defined in claim 3, wherein a capacitor is connectedin parallel with said Zener diode.

References Cited UNITED STATES PATENTS 3,380,796 4/ 1968 Kompelien 43169X 3,364,972 1/ 1968 Deubel et a1. 43169 FOREIGN PATENTS 940,836 11/ 1963Great Britain 431-69 CHARLES SUKALO, Primary Examiner W. C. ANDERSON,Assistant Examiner

